One known method for training an animal not to leave a prescribed area is to give the animal an electric shock as the animal approaches the boundary line defining the perimeter of the prescribed area. There are several known devices which apply an electric shock to train an animal. Some operate manually under the direct supervision of a trainer and others operate under radio control or in response to a particular behavior of the animal.
A system which operates to apply an electric shock to an animal based upon sensing the intensity of an electric field generated about a current carrying wire conductor surrounding the area in which the animal is to be confined is taught in U.S. Pat. No. 3,753,421. In accordance with the teaching in this patent, an electric antenna in the form of a wire is used to define the perimeter of the restricted area. AC current is passed through the wire to generate an electromagnetic field around the wire at sub-broadcast band frequency. A receiver is attached to the collar of the animal tuned to the frequency of this field. The receiver has an electric circuit which senses the intensity of the electromagnetic field and generates a high voltage when the detected signal strength exceeds a predetermined level corresponding to a desired fixed distance from the wire. The high voltage causes an electric shock.
The system described in the aforementioned patent suffers from a number of significant deficiencies which restrict its usefulness in terms of both its operation and practicality. One principal drawback of the system places a limitation on the geometry of the restricted area to basically that of a circle. The reason for this is due to how the system operates. The receiver circuit senses the level of field strength, i.e., the intensity of the field at the location of the receiver. Since the receiver is attached to the animal an electric shock will be generated each time the detected intensity is above a predetermined magnitude. Accordingly, the system is very sensitive to the magnitude of the electric current in the wire. If the area defined by the electrical wire is not circular but is rather of an irregular shape, the electromagnetic field intensity will not be uniform along the wire. As a result, the field strength will vary in magnitude particularly around sharp turns and corners or where the wire is looped close to itself. Since the receiver operates in response to field intensity, a variation in signal strength will vary the distance at which the receiver generates the electric shock. For a rectangular geometry, the electromagnetic field from the sides are additive thereby substantially doubling the signal strength near the corners. The field strength is thus shape dependent and for some irregular shapes the field may be caused to subtract near the wire conductor. Equally significant is the fact that the signal strength will also vary due to interference from any metal object lying in the vicinity of the wire. Moreover, since the magnitude of the electrical current in the wire controls the intensity of the field, the operator must also be careful to maintain the applied power to the wire at a constant level.
In addition, the system taught in the aforementioned patent operates to exponentially increase the generated voltage as the animal continues to approach the wire which exponentially increases the shock level. In practice, the animal almost momentarily receives a full power shock unless the animal is slowly walking toward the wire. A sudden application of a full shock operates as a severe punishment which bewilders the animal and leads to a confused state of behavior.